This invention relates to the continuous casting of molten metal in a mold.
A continuous casting process transforms molten metal into ingot for subsequent working such as by rolling or extrusion forming. The continuous casting process takes molten metal and pours it into contact with a mold which typically is water-cooled to extract heat from the molten metal through the wall of the mold. In this way, the outer part of the molten metal cools and solidifies into a shell, the shell further cooling and forming as it withdraws from the mold to form a solid, continuously formed ingot.
Metal casting processes in general have always required a lubricant for separating cast metal from a mold surface. Lard oil was commonly used as a mold lubricant for aluminum ingot casting until the mid-1950s. The lard oil was applied to molds by brushing or swabbing prior to the casting operation. Lard oil had the principal disadvantage of hardening to a highly viscous, grease-like consistency at approximately 40.degree. F. This grease-like form interfered with continuous casting methods where free-flowing lubricant is required. Further, the grease-like lard oil would build up on molds and interfere with ingot cooling.
As continuous casting became the accepted method for forming ingot, castor oil replaced lard oil as the most commonly used mold lubricant. Castor oil is obtained from pressing seeds of the castor plant. Typically, castor oil contains a predominant amount of the triglyceride of ricinoleic acid (12-hydroxyoleic acid). The remaining portion of the castor oil comprises mixed triglycerides of oleic, linoleic, and stearic acids. Castor oil thus falls in a chemical classification known as fatty oils. These materials, as a class, are practically insoluble in water and dissolve freely in organic solvents. The double bonds in hydroxyl groupings in castor oil produce many kinds of chemical reactions to form a wide variety of compounds.
Castor oil does not have the grease-like consistency of lard oil at just below room temperature. However, castor oil is very viscous and difficult to apply to molds in a uniform fashion, especially in cold weather operation. Castor oil undergoes polymerization under casting conditions and produces a varnish-like film on the mold and the ingot. This varnish-like film produces tears and unsatisfactory surface characteristics in the ingot. Further, in direct chill casting by water, castor oil does not separate from the cooling water easily to avoid contamination of the discharged water.
The disadvantages of castor oil used as a mold lubricant in continuous casting have encouraged the search for a replacement mold lubricant.
Smith et al., U.S. Pat. No. 3,524,751, discloses an aluminum ingot casting lubricant of 60%-80% castor oil and 40%-20% of an alkyl ester of an acetylated hydroxy fatty acid. The sole Example in Smith et al. mixes 75% castor oil and 25% n-butyl acetyl ricinoleate.
Gardner et al., Canadian Pat. No. 925,070, discloses a mold lubricant of polybutene alone and mixtures of polybutene in a predominant amount with vegetable oil, animal oil, or mineral oil.
It is an object of the present invention to provide a lubricating process for continuously casting molten metals.
It is another object of the present invention to provide a mold lubricating process which performs efficiently at reduced flow rates of lubricant over the mold.
It is a further object of the present invention to provide a mold lubricating process for casting aluminum and aluminum alloy.